This invention relates generally to internal cooling in gas turbine devices and more particularly to removing contaminants from the cooling medium prior to its entry into gas turbine cooling passages.
The traditional approach to cooling gas turbines has been to extract air from the gas turbine compressor and direct the air to the rotating and static turbine parts which are immersed in the hot combustor discharge gas. This has typically been accomplished, particularly for rotating part cooling, in an open cycle in which the cooling air is routed through the interior of the part (such as a turbine blade) and is then discharged into the working fluid. However, requirements for high efficiency and output in modern gas turbines have resulted in significant increases in operating temperatures which present a need for improved gas turbine cooling. Thus, it has become desirable to employ closed cycle cooling using steam as a cooling medium because its absorbed heat can be recovered through expansion in a steam turbine. Steam is readily available at several temperature and pressure levels in a combined cycle operation. However, the use of steam as a coolant could be limited by the tendency of solid particle contaminants in the steam to deposit in areas of high heat transfer, causing coolant flow blockage and metal overheating. Similarly, other coolants, such as air or Helium, could have corresponding problems due to solid impurities. The small cooling passages formed in the rotor blades are particularly susceptible to such plugging because the high centrifugal force drives the contaminants to the outer surfaces of these passages very quickly.
Therefore, it is necessary to clean the cooling medium as well as possible prior to entry into the cooling passages. Known techniques for removing particulates from fluids include filtration, magnetic separation and electrostatic precipitation. Filtration systems capable of achieving adequate filtration offboard are available, but the high pressure drop produced by such systems would compromise overall plant performance. Magnetic separation only removes ferromagnetic materials effectively and any momentary failure of the magnet would release contaminants into the gas turbine. Electrostatic precipitation has been found to be impractical for cleaning such coolants because operation of charged media at high pressures has been found to be difficult.
Accordingly, there is a need for reliably removing contaminants from gaseous coolants without producing a significant pressure loss.